Natural products contain a wide variety of functional groups with varying reactivity. These moieties include some that are nucleophilic (amine, thiol, etc.) and others that are electrophilic (ketones, aldehydes, epoxides, lactones, etc.). There are a number of techniques developed for the isolation of natural products that contain these reactive functional groups. For example, the chemoselective purification of natural products that contain the hydroxyl or the carboxylic acid functional group has been described. However, these functional groups are highly prevalent in a crude natural product extract and upon purification a large pool of molecules still remains. Also, the hydroxyl and carboxylic enrichment tags may find less utility for the discovery of unknown molecules.
There are fewer methods, especially chemoselective techniques, which can target less reactive functional groups such as alkenes and alkynes. For example, the alkyne moiety has only been identified in a few natural product classes, and is predominantly produced by plants or marine organisms. There are a number of biologically interesting natural products that contain a terminal alkyne moiety. Alkynes are often in very low abundance but are often important for the bioactivity of the natural product.
One method for their isolation is by cobalt complexes that can be formed with either terminal or internal alkynes with the complex subsequently disassociated by oxidizing reagents. This cobalt-based strategy has been utilized the enrichment of alkyne-functionalized glycolipids. Dicobaltoctacarbonyl can form complexes with both terminal and internal alkynes, often under thermal conditions. The resulting dicobalt complex can then be reacted with diphenylphosphine, which is linked to a solid support to facilitate compound enrichment. All molecules not containing an alkyne moiety can be rinsed away, and the cobalt-alkyne complex can be disrupted using an oxidizing agent, such as iron nitrate yielding the alkyne-containing compounds. However, such methods may lead to unintended transformations when the cobalt-alkyne complex reacts with diphenylphosine, the oxidation conditions required for alkyne release may be be harsh, and inorganic salt may make analysis of the enriched molecules by LC-MS difficult.
Alkynes also react with trimethylsilane (TMS). The acidic terminal alkyne hydrogen can be removed with a strong base and addition of chlorotrimethylsilane yields the TMS-protected alkyne. However, the oxophilic nature of silicon may prevent the capture of alkynes over hydroxyls or carboxylic acids, and the base required for the deprotonation of the terminal alkyne may be too strong and cause unintended degradation of labile natural products. Furthermore, traditional alkyne purification procedures may lead to degradation of reactive functional groups.
As such, there exists an unmet need for the development of a tag to enrich compounds containing this relatively uncommon moiety.
As described herein, it has been discovered that certain solid-phase azide supports find utility in the isolation of alkyne containing compounds from various sources.